1. Field of the Invention
The present invention relates to an ink jet recording apparatus and an ink jet recording method, and particularly relates to an ink jet recording apparatus and an ink jet recording method for recording a high-quality image without density unevenness.
2. Description of the Related Art
In a recording apparatus in which a recording head having a plurality of recording elements forms an image while carrying out main-scanning and sub-scanning relatively to a recording medium, there is a problem that density unevenness such as white stripes or black stripes extending in the main-scanning direction occurs periodically. The phenomenon that density unevenness occurs is called banding.
In an ink jet recording apparatus, ink is ejected from nozzles which are recording elements, and landed on a recording medium to thereby record pixels. Generally, in the ink jet recording apparatus, ink is ejected at the same time as main-scanning so that recording is carried out by unit of a belt-like area (hereinafter referred to as xe2x80x9cbandxe2x80x9d) long in the main-scanning direction. Such bands are lined up in the sub-scanning direction so as to form an image.
Particularly in the ink jet recording apparatus, banding caused by ink bleed is apt to occur, resulting in a problem in deterioration of image quality. Such banding caused by ink bleed occurs in a joint between bands when recording is performed with comparatively high density. The banding caused by ink bleed has a feature in that it appears as a black stripe with high density.
To cope with the banding, there has been proposed a technique in which the density of an image to be recorded is adjusted to prevent high-density stripes from occurring. JP-A-2-3326 discloses a method in which density adjustment is given to end portions of any band bordering adjacent bands. JP-A-2-3326 says that the reason why the density of a joint between bands increases is because the width of each recorded band spreads due to ink bleed. Then, when only a high-density area is selected and the density therein is reduced in accordance with a density correction curve shown in FIG. 17, the density can be prevented from increasing.
Further, JP-A-8-72312 explicates the mechanism causing increase in the density in band end portions, and discloses a method to reduce the density of the band end portions in accordance with the kind of recording medium. According to JP-A-8-72312, the density in the band end portions increases due to the influence of beading. When high-density recording is performed, ink heaps up in the surface of the recording medium immediately after ink drops are landed on the recording medium. At this time, the ink moves to band end portions 32 by the action of the surface tension of ink 8 as shown in FIG. 18. As a result, the density in the band end portions 32 increases.
As the method for adjusting the density in the band end portions, JP-A-64-26460 discloses a method based on forcible thinning out of recording-target pixels 22 as shown in FIG. 19. In this method, the recording-target pixels 22 on both sides of a band boarder 26 are thinned out alternately when they are recorded. As a result, the density in the band end portions 32 is lowered so that the banding becomes inconspicuous.
However, according to-the related-art-method for reducing the density in the band end portions, no sufficient effect to eliminate banding can be obtained because of the problem of the area to be reduced in density.
FIG. 20 is a graph showing the change of density near a band border when the related-art method was applied. The line A in FIG. 20 shows the case where the related-art method was not applied. On the other hand, there occurred a problem that a white stripe appeared additionally when the related-art method was applied (line B in FIG. 20). Because the white stripe appeared additionally thus, it was difficult to eliminate banding perfectly.
It is therefore an object of the invention to provide an ink jet recording apparatus and an ink jet recording method for performing high-quality recording without banding when the mechanism causing occurrence of banding is explicated, an area having increase in density is specified, and the density in the specified area is adjusted.
An ink jet recording apparatus according to the invention includes a recording head having a plurality of recording elements arrayed for ejecting ink drops including color materials to record recording pixels, a recording element control unit for controlling the ejection of the ink drops from the recording elements, a main-scanning unit for making the recording head relatively main-scan a recording medium in a first direction while the recording head faces to the recording medium, a sub-scanning unit for making the recording head relatively sub-scan the recording medium in a second direction while the recording head faces to the recording medium, and a scanning control unit for controlling the main-scanning unit and the sub-scanning unit. The first and second directions are different from each other. The recording elements are actuated with the main-scanning to record bands in which a longitudinal direction thereof is a main-scanning direction. An image is recorded by repeating the main-scanning and the sub-scanning. The recording element control unit has a recording density control unit for reducing recording density only in an area near a border portion of the bands adjacent to unrecorded areas of border portions of the bands as long as the recording density is high.
The present inventor find that an area where density increase occurs as banding does not center about a band border 26 practically, but is displaced toward one of the bands on either side of the band border 26. The inventor further proves that when recording is carried out with high density, an area where the density increases to produce a black stripe is limited to an area which is a band end portion and in which a band adjacent to the band end portion is unrecorded.
Accordingly, the area to be reduced in recording density by the recording density control unit is limited by a recording element control unit to an area whose adjacent area is unrecorded. Thus, black stripes can be prevented from occurring. In addition, there is no fear that recording density is reduced in any unnecessary area so that an image having no banding and without occurrence of any white stripe can be obtained.